Field of the Invention
The invention relates to a nuclear magnetic flowmeter for determining the flow of a medium flowing through a measuring tube, having a magnetic field generator consisting of permanent magnets for generating a magnetic field interfusing the medium over a magnetic field section, having a pre-magnetization section located within the magnetic field section and having a measuring device also located in the magnetic field section LM including a coil-shaped antenna with the length serving as a measuring antenna. Furthermore, the invention relates to a method for operating a nuclear magnetic flowmeter.
Description of Related Art
The atomic nuclei of the elements having nuclear spin also have a magnetic moment caused by nuclear spin. Nuclear spin can be regarded as angular momentum describable by a vector, and accordingly, the magnetic moment can also be described by a vector, which is aligned parallel to the vector of the angular momentum. The vector of the magnetic moment of an atomic nucleus, in the presence of a macroscopic magnetic field, aligns itself parallel to the vector of the macroscopic magnetic field at the location of the atomic nucleus. The vector of the magnetic moment of the atomic nucleus precesses around the vector of the macroscopic magnetic field at the location of the atomic nucleus. The frequency of precession is the Larmor frequency ωL and is proportional to the magnitude of the magnetic field strength B. The Larmor frequency is calculated according to ωL=γ·B, with γ being the gyromagnetic ratio, which is at a maximum for hydrogen nuclei. The gyromagnetic ratio indicates the proportionality factor between the angular momentum or the spin of a particle and the associated magnetic moment.
Measuring and analyzing methods that utilize the precession of the atomic nuclei with a magnetic moment in the presence of a macroscopic magnetic field are referred to as nuclear magnetic resonance measuring or analyzing methods. This is called nuclear magnetic resonance (NMR).
A requirement for an analysis of a multi-phase medium using nuclear magnetic resonance is that the phases to be analyzed of the medium are able to be excited to distinguishable nuclear magnetic resonances. The analysis can include the flow velocities of the individual phases of the medium and the relative proportions of individual phases in the multiphase medium. Nuclear magnetic flowmeters can, for example, be used for analysis of multiphase mediums extracted from oil sources. The medium, then, consists essentially of the phases crude oil, natural gas and salt water, wherein all phases include hydrogen atom nuclei.
The analysis of the medium extracted from oil sources can be performed using so-called test separators. These divert a small amount of the extracted medium, separate the individual phases of the medium from one another and determine the proportions of the individual phases in the medium. However, test separators are not able to reliably measure crude oil proportions of less than 5%. Since the crude oil proportions are continuously sinking in all sources and the crude oil proportion of many sources is already less than 5%, it is not possible at this time to economically exploit these sources using test separators. In order to further exploit sources having even a very small crude oil proportion, accordingly exact flowmeters are necessary.
Normally, electric signals induced in a measuring antenna from the precessing atomic nuclei after excitation are used as the dependent variable for evaluation. A requirement for the measurement of a multi-phase medium is, as described above, that the individual phases of the medium can be excited to distinguishable nuclear magnetic resonances. The magnitude of the electric signal induced in the measuring antenna from the precessing atomic nuclei of a phase of the medium is dependent on the number of precessing atomic nuclei per volume element in this phase, hence dependent on the density of the phase, but also on the impact time of the atomic nucleus in the influencing magnetic field. Consequently, the magnitude of the induced electric signal is different for each phase of the medium.
Measuring methods for determining the individual phases of the medium provide that the medium is exposed to the magnetic field generated in the pre-magnetization section for a certain time, and then, the magnetization of the medium in the direction of the magnetic field is determined after different lengths of exposure of the magnetic field generated in the pre-magnetization section on the medium. Determining the magnetization of the medium after a certain impact time occurs in the measuring device by exciting the magnetized medium with excitation signals, measuring the measuring signals caused by the excitation signals in the medium and evaluating the measuring signals.
Nuclear magnetic flowmeters known from the prior art of the type described in the introduction vary the effective impact time of the magnetic field on the medium by changing the magnetic field, wherein the changing of the magnetic field is caused by a mechanism.
A nuclear magnetic flowmeter of the type described in the introduction is known from U.S. Pat. No. 7,872,424. The magnetic field generator includes several consecutive magnet arrangements arranged around the measuring tube along the longitudinal axis of the measuring tube. Each of the magnet arrangements is turnable around the longitudinal axis of the measuring tube and interfuses the medium flowing through the measuring tube with a magnetic field demonstrating a certain direction. The effective pre-magnetization section is then varied in that each of the magnetic fields of the individual magnet arrangements are aligned parallel or antiparallel to one another. In a parallel alignment of two magnetic fields each generated by one magnet arrangement, the magnetization in the medium builds up over the time, it takes, until the medium has flowed through both magnet arrangements. In an antiparallel alignment of two magnetic fields the magnetization in the medium builds up in the first magnet arrangement and is destroyed in the second magnet arrangement due to the opposing field direction in an antiparallel alignment of two adjacent magnetic fields. In this case, the effective pre-magnetization section is zero.
Turning each magnet assembly requires a mechanism. This mechanism requires, on the one hand, space, and on the other hand, is associated with costs. Additionally, mechanically moving parts are subject to normal wear and tear and need to be maintained on a regular basis. This means efforts in both time and cost.
A device for varying the pre-magnetization section is also known from the prior art, in which several magnet arrangements are arranged around the measuring tube. Each of these magnet arrangements consists of an inner ring of a permanent-magnetic material and an outer ring also of a permanent-magnetic material. Each of these rings generates a magnetic field. Both rings can be shifted relative to one another. If the rings are located in a position relative to one another so that both magnetic fields are aligned parallel to one another, then there is a strong magnetic field within the magnet arrangement. If the two rings are aligned relative to one another so that the two magnetic fields are antiparallel to one another, then the field within the magnet arrangement is zero. By arranging several such magnet arrangements consecutively, the effective pre-magnetization section can be arbitrarily varied.
Here, the pre-magnetization section is also set by a mechanism which turns each of the rings of a magnet arrangement in opposite directions. This requires a generous amount of time and the moving components require maintenance on a regular basis, which is associated with a higher investment in time and costs as well as wear.